A wonderful example of this is The Wand of the War Mage, ph 212 Basic Rules.
"While holding this wand, you gain a bonus to spell attack rolls determined by the rarity of the wand, and you ignore half the deck when you make a spell attack.
rare (+1), rare (+2) or very rare (+3) "
There are multiple elements already incorporated in the game that give Focus more power than a traditional Focus would normally do. For example: A Warlock (once he has taken the Enhanced Covenant Weapon feature) and the Bard Swords College can use their melee weapons as a focus to cast spells. Clerics and paladins can use their shields as spell casters. All of these elements can be magical in nature and potentially give advantages to your spell casts and / or attack / damage rolls. But each class can have some kind of focus or weapon / empowered element that can be used as a focus, not everyone will make spells, but many can. If there is something more specific you are looking for, I would recommend reading carefully the items that are currently manufactured within 5E through D & D Beyond or other books organized at your disposal.